Pile driving system



March 3, 1970 A. lovls PILE DRIVING SYSTEM 2 Sheets-Sheet 1 Filed Dec.5, 1967 March 3, 1970 A. Jovls 3,498,388

n PILE DRIVING SYSTEM Filed Dec. 5. 1967 2 sheets-sheet 2 7 FWZAOOELEPOMETEP OUTPUT United States Patent O 3,498,388 PILE DRIVINGSYSTEM Arthur Jovis, New York, N.Y. (1501 Underclitf Ave., Bronx, N.Y.10453) Filed Dec. 5, 1967, Ser. N0. 688,184 Int. Cl. E21c 3/24; E02d5/34; G01n 3/42 U.S. Cl. 173-2 9 Claims ABSTRACT OF THE DISCLOSURESystem: automatically controlling the pile driver, utilizing EngineeringNews formula 2E (P S-i-C marking the penetration per hammer blow on thepile; and keeping a permanent record of operation.

This invention relates to a system including methods and apparatus tocontrol the operation of steam or compressed air (fluid driven) piledriving hammers.

Among the principal objects of the present invention are the provisionof structure for:

l) Maintaining at a constant rate the energy delivered to the pile bythe pile hammer.

(2) De-activating the pile hammer when the pile has been driven to apre-determined driving resistance; such driving resistance being ameasure of the load bearing capacity of the pile.

(3) Providing a tangible and permanent record of the magnitude ofsuccessive pile displacements, and hence of the bearing capacity of thepile.

(4) Maintaining uniformity of settlement of each pile foundation inrelation to the other pile foundations in the same project. Suchsettlement occurs when the superimposed load is applied to the pilefoundation.

When a pile is driven into the earth varying factors which affect theload bearing capacity of the pile are met with. Mostly these factors aredue to differing types of soil, variations in successive earth strata,and generally the total frictional resistance to the penetration of thepile into the earth. These various factors express themselves as arelation of the axial movement of the pile to the energy supplied to thepile by the pile hammer.

Various formulae have been proposed to determine the load carryingcapacity of piles, but with the exception of the Engineering Newsformula (which directly relates energy to axial movement) they have beenfound impractical and have been discarded.

The Engineering News formula S-i-C was proposed by A. M. Wellington in1888. While it is partly empirical, it nevertheless has stood the testof time, and has been almost universally adopted by the engineeringprofession.

In the formula:

In structural engineering, it is the practice to design a group of piles(known as a pile cluster) to support a known load, and to assign to eachpile in the cluster its proportional share of the total load.

ice

From examination of borings, determination of test pile loads, type andsize of pile etc., the engineer establishes an estimate of the safe loadcapacity of a pile. When this value is assigned to P in the EngineeringNews formula all factors in the formula, with the exception of S, becomeconstants. Solving then for S We find:

Having previously established a value of P for a specific project andknowing the value of E for the speed and particular lmake and modelhammer used in the operation, the field engineer will observe the piledriving operation and when the desired value of S (i.e. the pilepenetration per hammer blow) is reached, the driving is stopped.

Driving a pile to a greater penetration per blow than prescribed (i.e.over-driving) can cause damage or crippling of the pile; driving to alesser value of S will not develop the required bearing value of thepile and may result in undue settlement or even failure of the pile.

Any foundation not resting directly on bed rock will have some degree ofprimary settlement, as the load of the structure is applied. Thissettlement is unavoidable and is not harmful provided that it isself-limiting and that the amount and rate of settlement is uniformthroughout the structure; otherwise serious damage to, or even failureof, the superstructure can result.

In the case of pile foundations, the primary settlement of thefoundation units will be in direct relation to the values of S in theformula. In order therefore, to maintain the required uniformity ofsettlement it is important that during the driving operation the finalaverage of the penetration of the pile per blow of the hammer should bein conformity with the computed value of S for all the piles comprisingthe substructure. It is one of the objects of this invention to providesuch required uniformity of penetration per blow, and thus provide auniformity of structural settlement.

When the Engineering News formula first came into use, the drop hammeroperating at a rate of l0 to 20 blows per minute was the only type ofpile driver in general use, and it was then a simple matter for the eldengineer to determine the value of the component factors in the formula,and also to determine by observation and experience at which point tostop driving the pile so as to achieve the two partly conflictingobjectives of developing the optimum load bearing capacity of the pile,while avoiding over-driving and resulting damage to the pile.

The introduction and development of the uid driven pile hammer operatingat a rate of to 275 or more blows per minute, has introduced the problemof operational control.

Due to the speed at which a modern power hammer operates, it isvirtually impossible for the field engineer to simultaneously:

(a) Determine at what point the pile has been driven to the requiredoptimum bearing capacity and to deactivate the driving at this point;

(b) Maintain energy supplied by the pile 4hammer to the pile at aconstant value; and

(c) Count the number of blows per minutes struck by the pile hammer andthereby determine (as explained hereinafter) the value of E in theformula.

The energy delivered bya fluid driven pile hammer is a function not onlyof the weight of the hammer and its length of travel (which factors areconstant for a given hammer) but it is also a function of the meaneffective pressure acting upon the piston in the hammer drive.Calculations based on steam or air pressure are misleading because notwo set-ups are identical and it is impractical, if not impossible, todetermine the mean effective pressure in the working cylinder from thepressure shown on the boiler gage. Even if this were not the case,variations in boiler pressure would prevent maintaining a constant rateof delivery of energy by the pile hammer.

Because of the aforementioned difficulties, manufacturers have conductedexhaustive tests using -highly -sophisticated test equipment and havecarefully determined the energy delivered by the hammer at the point ofimpact, as a function of the operating rate of the hammer. The resultsof such tests have been tabulated by the respective manufacturers andare used in the eld to determine the value of E in the Engineering Newsformula.

The following is an excerpt from a table furnished by a manufacturer ofpile driving hammers and states the value of E as a function of strokesper minute for a. particular model of this manufacturer.

If a constant rate of operation is maintained, and if this is accuratelydetermined, the value of E in the formula can be established by the useof this table Without recourse to measurements of fluid pressure.

As the driving of a pile progresses the resistance to penetrationincreases; this results in an increase in the rebound of the hammer andtherefore in its rate of operation.

It is apparent that unless some means is introduced for compensating forvariations in operating rate resulting from variations in iiuid pressureand in elastic rebound, said E in the formula will not remain constantand that its proper value cannot be applied in the present state of theart.

A further object of this invention is to facilitate use of theEngineering News formula in connection with the operation of fluiddriven pile hammers 'by making the values of E and S, as defined above,readily determined factors. This is accomplished by use of apparatuswhich makes E a constant (by holding the hammer at a xed operatingspeed) irrespective of changing operating conditions, and which providesa .means for accurately measuring S and uses the magnitude of S tode-activate the vpile hammer at the appropriate point. The controlsystem involved in this invention attains these objectives and providesa permanent record of the factors E and S.

These objects and other incidental ends and advantages will more fullyappear in the progress of this disclosure andy be pointed out in theappended claims.

In the drawings:

FIGURE 1 is a schematic block diagram of a preferred embodiment of theinvention.

FIGURiE 2 is a graph of accelerometer output.

FIGURE 3 is a graph of rectifier output.

FIGURE 4 is a graph of monostable multivibrator output.

FIGURE 5 is a graph of lter output.

FIGURE 6 is a graph of iirst integrator output.

FIGURE 7 is a graph of change in multivibrator output in relation tohammer blow rate.

FIGURE 8 is a graph of pile penetration in relation to the presentsystem.

FIGURE 9 is a fragmentary elevational view showing one form of pilepenetration marking.

FIGURE 10 is a fragmentary elevational View Showing another form of pilepenetration marking,

Turning to FIGURE l, this diagram is also a flow chart of the system,generally indicated by reference character 10. Elements 11 to 17inclusive constitute the speed control while elements 18 through 23,cooperating with element 17, constitute the turn-off control andtangible recording means. Turning to the speed control, which in thissystem is synonymous with energy control, my control compares the actualrate of which the hammer 31 delivers blows to a pile 30 with apredetermined rate reference. An electronically controlled feed backadvances or retards the energy supply throttle 17 so as to maintain aconstant rate of operation of the hammer 31 as dictated by the referencesignal, thereby holding constant the energy delivered to the pile by thepile hammer.

The system involves a means responsive to change in position of, orapplication of force, to the pile, so that at each blow of the pilehammer said means produces an output signal; said output signal is fedto a comparison means, which compares said output signal with referencesignal corresponding to the desired operating speed, and produces anoutput related to the difference between reference signal and the signalfrom the means responsive to strokes of the pile hammer; said output ofthe comparison means is amplified to the extent necessary to drive aconventional pile hammer speed control means, typically an electricallycontrolled pressure throttle.

I have referred to the various means to be used, in general-terms. Thusthe means responsive to changes in pile position or to application ofpile hammer force, could be optical, mechanical, magnetic,electrostatic, and so forth. The output of such means could be comparedwith a reference by measuring phase shift or time lag between them,.by adigital comparator, and so forth; of course, different comparison meanscall for different appropriate reference means. I turn now to mypreferred system, although I desire to claim my invention in the broaderterms I have used above.

' My preferred system uses an accelerometer 11 as the force or positionchange responsive means because such a means is useful in connectionwith the other aspects of my control system described subsequently. Mypreferred comparison method, described below in more detail, uses theacelerometer 11 to drive electronic circuitry which produces as itsoutput a slowly varying DC signal, the amplitude of which isproportional to the frequency with which the hammer 31 strikes the pile30 (pile hammer frate); and then compares such DC signal with areference DC signal proportional to the desired rate, by means of adifferent amplier 15.

'Ihose familiar with the art will recognize that the system is neitheranalog nor digital, but a hybrid of both systems. One reason I havepreferred this partially digital control approach relates to anotheraspect of my invention. I have avoided tachometer and similar analogspeed control systems that require additions to, or attachments to themain part of the drive system. My system minimally affects the main partof the hammer system and is thus more flexible than a system which wouldrequire extensive modification of existing qeuipment.

In FIGURE 1 a conventional accelerometer 11 is of the crystal,semiconductor, or magnetic type. It is preferably fastened to the pile30 by bolting it into the drive pile cap 32, or other such means ofattachment as will make it move in unison with the pile, with outputshown in FIGURE 2. Rectifier 12 is a conventional circuit. Its input isthe output of accelerometer 11 and it prepares the output of theaccelerometer for use by the monostable multivibrator 13` by removingeither the positive or negative portion of the signal (FIGURE 3) asappropriate for the type of multivibrator used. Rectifier 12 may alsoinclude a biasing means so that it passes only those signals of aspecified polarity which exceed a specied threshold value. Inconventional transistorized circuitry commer1 cially available at thistime, the foregoing rectification and biasing are included as part ofthe package comprising the monostable multivibrator 13.

The monostable multivibrator 13, a conventional device which byresponding to each input pulse delivered to it, in excess of a certainthreshold value, produces at its output a substantially rectangular waveor pulse of fixed duration and fixed amplitude (FIGURE 4). The durationand amplitude are fixed by conventional design procedures, as is thethreshold. It is customary to refer to the multivibrator as on duringdelivery of an output wave and off at other times, when it is quiescent.In the present system, the multivibrator is on immediately followingeach pile hammer blow; it remains on for a length of time determined bythe designer of the circuit, and then returns to the off state. It isreadily seen that for any given circuit the percentage of on time isdependent on the rate of operation of the hammer; the more frequentlyblows are delivered, the higher the percentage of time the multivibratoris on, and therefore the average value of the said output signal will bedirectly proportional to operating speed (see FIGURE 7). FIG- URE 7 is agraphic illustration of the increase in the average value of the outputsignal of the multivibrator 13 in relation to blows per second of thepile hammer 31, the dotted line 33 in FIGUR-E 7 indicating the averagevalue of the output signal of the multivibrator 13.

In designing the multivibrator, caution must be taken that the durationof the output pulse selected be less than the minimum time possiblebetween successive input pulses to the multivibrator. Otherwise therewould be an overlapping of output pulses and the operation describedhere would be unsatisfactory. In the case of a pile hammer operating at150 blows per minute, there is approximately 0.4 second between thebeginning of successive strokes. Accordingly, multivibrator pulseduration must be less than 0.4 second; a suitable value would be about0.2 second, making the multivibrator on about 50% of the time at 150blows per minute.

The pulse (digital) output of multivibrator 13 is converted to a slowlyvarying DC (analog) signal by the filter 14, a conventional low-passfilter (FIGURE 5). This output signal will be proportional to on,time orhammer speed. The filter should have as short a time constant aspossible without making the system unstable; this determination can bemade by conventional servomechanism design techniques.

The output of filter 14 is one of the two inputs to the differentialamplifier 15 of conventional type. The other input to amplifier 15 isthe speed reference 16 a conventional reference circuit, such as theoutput from a precision potentiometer connected across a Zener diode, orstandard reference cell. The setting on the potentiometer is adjusted bythe field engineer to correspond to the desired operating speed (rate),as explained below. The difference amplifier produces an output which isproportional to the difference between its two inputs. By suitablyproportioning the value of the signal from speed reference 16, theoutput of the difference amplifier will be proportioned to thedifference between actual operating rate of the hammer and the desiredrate.

For example, in the case referred to above where the hammer is assumedto operate at 150 blows per minute and the multivibrator 13 is designedto be on 50% of the time; if the amplitude of the multivibratorrectangular output wave (FIGURE 4) is 1 volt, the4 average value of thesignal will be 0.5 volt. This will be the reference voltage, assuming afilter transfer characteristic of 100%. Any difference in the transfercharacteristic will of course require corresponding adjustment in thereference voltage.

The operation of the circuit is illustrated by example. As theresistance to the driving of the pile increases, the hammer rebound alsoincreases, causing an increase in the rate of operation of the hammer.Assuming a 5% increase in this rate of operation from 150 to 1571/2blows per minute, the averaged signal will increase from 0.5 volt to0.525 volt. The difference amplifier will then produce an output equalto .025 time its gain.

The output of the differential amplifier 15 is used to operate the pilehammer throttle. It is so connected to the throttle that the throttleopening is reduced if actual speed exceeds reference speed and thethrottle opening is advanced if' actual speed falls below referencespeed. The higher the gain of the amplifier the more sensitive thesystem is, but the greater the danger of instability. These two factorsare accommodated by conventional design techniques.

I turn now to that aspect of the system which deals with the measurementof displacement and the use of such data to de-activate the pile hammerat an appropriate point. My method of control develops a signalproportional to S of the Engineering News formula and compares it with areference proportional in the same ratio to the precalculated value ofS, which when substituted into said formula will give the desired valueof P, the predetermined load bearing capacity of the pile.

When S, as measured by my control system, falls to the value of S asprecalculated, my system 10 closes the pile hammer throttle 17. Thisaspect of the system has four parts: (l) a displacement measuring meansresponsive to the magnitude of change in position of the pile or afunction thereof (such as first or second derivative of position); (2)internal computation means receiving as its input the output of saiddisplacement measuring means and capable of producing as an output asignal proportional to the sum or average of successive changes in theposition of the pile for a specified number of successive displacementsor during a specified time interval corresponding to such number ofdisplacements; (3) a reference means capable of variation or adjustmentto correspond to a predetermined value proportional to the value of'Swhich corresponds to the desired bearing strength; (4) a comparisonmeans capable of comparing the magnitude of said reference and theoutput of said internal computation means, and capable of deliveringsuliicient output to actuate a means for stopping the operation of thepile hammer (e.g., a relay to close the throttle) when such comparisonindicates equality ofthe two signals being compared.

I have referred to the various means to be used in general terms, asbefore. I turn now to my preferred system, although I desire to claim myinvention in the broader terms I have used above.

Reference is made to FIGURE l. The accelerometer 11 is the sameaccelerometer previously used. Its output is integrated twice by firstintegrator 18 and second integrator 19 (or by a double integrator), sothat the output of the first integrator 18 measures the velocity of thepile (see FIGURE 6) and the output of the second integrator 19 measuresthe position of the pile (since a=d2s/dt2).

See FIGURE 8 which is a graph of second integrator output, and stripchart and comparator input, and where horizontal dot-dash line 33indicates the critical value of penetration or penetration ratereference, below which the required value of S is achieved, verticalline 34 is a scale of pile penetration in inches, and horizontal line 35is a scale of hammer blow rate (time interval). At the peaks 36, 37 and38 the gate 24 triggers reset to zero at vertical lines 46, 47 and 48every N hammer blows. At peak 39 the gate 24 triggers to zero reset atline 49 and comparator 23 closes throttle 17 since 2NE P The output ofthe monostable multivibrator 13 is used to drive reset circuitry 20 usedin conjunction with the integrators 18 and 19. The reset circuitry 20serves several functions, some beneficial and some essential. In thefirst place, the reset circuitry resets the first integrator after everyintegration or after a specified number of blows; this is a rezeroingoperation which improves the accuracy of integration and decreases theeffect of drift in lower quality equipment. Second, the reset circuitryresets the second integrator after every nth integration, where N is thenumber of blows over which S of the Engineering News formula is to beaveraged, e.g., 5, 8, 101, etc. (FIG- URE 8). The rezeroing alsodecreases drift in this integrator as a byproduct of its principalfunction. Third, the reset circuitry opens an electronic gate 24 andcauses the following element, comparator 23 to operate after every nthblow just prior to the resetting of the second integrator to zero. Thisreset circuitry 20 is essentially a commercially available scale-of-veor scale-of-eight, etc., counter.

The penetration reference 22 is a reference voltage, similar to thespeed reference 16, proportional to the desired value of S. Thecomparator 23 compares the said reference with the sum of n displacement(equal to 11X the average displacement, S). When the gate 24 causescomparator 23 to compare S with the reference, comparator 23 doesnothing if the actual S exceeds the reference value, but it turns offthe throttle 17 if Sn equals or falls below the reference. Immediatelyafter each comparison (unless the device is turned off), the resetcircuitry 20 resets the integrators 18 and 19 for a new accumulation ofS and new comparison.

The strip chart recorder 21 is a recording device, such as anoscillograph or strip chart recorder, which makes a permanent record ofS for such future use as may be advisable or required by the conditionsof operation.

I have indicated that the above system with elements 158, 19, and 24 asdescribed, is my preferred embodiment. It should be noted that, when Sis averaged over eight blows, it is possible for the hammer to delivermore blows than absolutely necessary, perhaps seven additional blows andprobably three or four. These extra blows are not usually objectionable,although an undue number of them can in some circumstances damage thepile. An alternative system may be used which averages S continuously,instead of after every nth blow.

In the alternative system the gate 24 is eliminated along with itsinputs and output, and the reset circuitry 20 serves only to eliminateundue drift. An additional gate 24 is connected with its input at theoutput of the first integrator 18 and its output at the input of thecomparaor 23. The new element is a moving-time integrator or averager(Reference: Philbrick Researches, Inc., Applications Manual forComputing Amplifiers (2d ed. 1966), p. 76; Hansen, New Approaches to theDesign of Active Filters, The Lightning Empiricist, 1965, vol. 13, pp.11, 13 (Part I) whose output is em=input voltage T0=averaging time Sucha circuit produces an output which at any instant is the definiteintegral of its input for the last To. seconds; this in turn is equal tothe average input times the number of inputs during the interval of To.It is readily seen that such circuitry is capable of giving theinstantaneous value of S (multiplied by a proportionality constant). Ina system embodying such circuitry, no gating of the cornparator isnecessary since the output of gate 24 is always S (times a constant),unlike the output of the second integrator 19 above, which is S (times aconstant) only at the close of each cycle of n integrations, just beforereset takes place.

Although the foregoing system permits instantaneous and continualcomparison of actual S with the reference, it is not my preferredembodiment because the circuitry is more complex and the determinationof appropriate constants more difficult. In some applications, however,the added complexity may be warranted.

Turning to FIGURE 9 and 101 alternative or supplemental recording meansfor the determination of the value of S (the penetration of the pile perblow of the hammer)v is provided by solenoidal activated markers 51 and61 (FIGURES 9 and l0). The solenoids 52 and 62 are powered by the outputof the monostable multivibrator 13 the signal being properly amplifiedor otherwise modified.

These markers when operated in conjunction with wood piles in the caseof marker 51 has a sharply pointed armature 53 which is driven againstthe pile 30 co-ordiuately with each blow of the pile hammer 31 therebyproducing an indentation 54 on the pile. Each indentation corresponds toone blow of the hammer, the space between each successive indentationcorresponds to a value of S. T o obtain an average value of S for Nblows, the total measurement taken from the last mark on the pile to thepreceding (N-I-1)th mark divided by N gives the desired value of S.

Since the pointed marker is ineffective against a steel pile 40 apressured paint spray 66 with the spray valve 65 activated by thearmature 63 of the solenoid -62 (FIG- URE 10) is more suitable. Thisproduces a paint mark 64 corresponding to each blow of the hammer. Inall other respects the determination of the value of S is the same asfor the pointed marker.

I wish it to be understood that I do not desire to be limited to theexact details shown and described for obvious modifications Will occurto a person skilled in the art to which the present invention relates.

I claim:

1. In a pile driving control system, having a throttle, the improvementcomprising: governing means for maintaining a constant rate of hammeraction connected to said throttle; and sensing means responsive tomovement of the pile, and connected to said governing means, whereby theenergy delivered by the hammer to the pile is maintained within closelypredetermined limits.

2. Structure in accordance with claim 1, said sensing means including anaccelerometer associated with said pile.

3. Structure as claimed in claim 1, in which the sensing means measuresacceleration of the pile per hammer blow, and means to convert saidacceleration measurement of the sensing means into a measurement of pilepenetration (S of the Engineering News formula) per hammer blow.

4. Structure in accordance with claim 2, said sensing means being anaccelerometer providing an output voltage in correlation to the pilemovement, means providing a reference voltage, and a comparator; saidaccelerometer voltage and said reference voltage being fed to saidcomparator, the output of said comparator being connected to controlsaid throttle, Awhereby when the voltage per pile movement is below thereference voltage the pile driver is deactivated.

5. Structure in accordance with claim 2 including means for visiblymarking the pile at each blow, said marking means being actuated by saidaccelerometer.

6. Structure as claimed in claim 3 having a recorder and in which theoutput of the means to convert is `fed to said recorder to provide atangible record of pile penetration per blow.

7. Structure as claimed in claim 3 having means to totalize the value ofa predetermined number of said measurements of pile penetration; meansto provide a penetration rate reference; means comparing said totalizedvalue with said penetration rate reference; and when the totalized valueis less than the reference value, to act on the throttle and therebydeactivate the pile driver, whereby in a plurality of piles, each pilehaving been driven to a uniform degree of penetration per hammer blow,supports an equal share of the load carried by all of the piles in saidplurality with consequent uniform settlement of a structure supportedthereby.

8. Structure as claimed in claim 3 including means to mark the pile perhammer blow, said pile-marking means being xed in position relative tosaid pile.

9. Structure in accordance with claim 3 including electrical gatingmeans for rendering said comparator operative at intervals correspondingto a group of hammer blows.

References Cited UNITED STATES PATENTS 2,580,299 12/1951 Hunicke 73-843,353,362 ll/1967 Lubinski 61-53.5

ERNEST R. PURSER, Primary Examiner U.S. Cl. X.R.

